For a number of years continuous casting of aluminum, lead, zinc, and the like has been conducted in commercial scale operations with continuous casters such as shown in U.S. Pat. Nos. 2,790,216 or 4,054,173. These patents are incorporated herein by this reference. Such a continuous caster comprises a pair of rotating water cooled rolls. Molten aluminum, for example, is fed into the nip of the rolls just prior to the line of closest approach of the two rolls. Heat is rapidly extracted from the molten metal by contact with the rolls and freezing occurs before the metal reaches the line of closest approach to the two rolls. In U.S. Pat. No. 4,054,173 substantial hot reduction of the thickness of the resultant aluminum sheet therefore occurs between the rolls for recrystallizing the sheet.
With such a caster aluminum sheets a couple meters wide and about one centimeter thick can be continuously cast at rates of more than a meter per minute for several days at a time. One important aspect of such a continuous caster is the structure for feeding molten metal to the nip of the rolls.
Reasonably satisfactory molten metal feed tips have been made from an insulating composition that is a composite of asbestos fibers and clay particles. Such a material is available from the Johns Manville Company under their trademark Marinite. This material is obtained in flat slabs or planks which are cut and machined to the desired configuration for molten metal feed tips, as well as various launders and other parts of the molten metal handling system.
Although such material is readily formed and resists molten aluminum and the like it has significant shortcomings. One of the more significant problems with such material is the presence of asbestos which is considered carcinogenic. Partial or absolute bans on use of asbestos-containing products have been proposed or are coming into effect. This is a particular problem with such materials for construction of molten metal feed tips for continuous casters since the practice has been to machine the material to the desired geometry. Machining produces many particles which when airborne are extremely hazardous. Expensive dust collection and recovery systems are mandatory for machining the asbestos-containing materials.
Current materials for feed tips are also somewhat fragile and difficulty is encountered with inadvertent breakage. Moisture is quite detrimental to the strength of the composite asbestos-clay material and can exacerbate the breakage problem. Moisture is commonly present in the vicinity of continuous casting machines since the rolls are water cooled, and surface release compounds are sometimes applied to the rolls from aqueous suspensions. In humid environments moisture can condense on the rolls and come in contact with the molten metal feed tips.
A molten metal feed tip made of the asbestos-clay composition can ordinarily be used for only a single run on a continuous caster. Breakage of such feed tips during cleaning and reconditioning for a new run is quite common and such feed tips must be continually replaced.
Further, because of the fragile nature of the material such a feed tip is fabricated to have the maximum thickness of material possible to maximize strength near the thin downstream edges of the tip that fit between the rolls. Thus, as seen in U.S. Pat. No. 4,054,173, the external faces of the continuous caster feed tip are commonly provided with a radius closely paralleling the radius of the rolls of the continuous caster. This can aggravate the problem of moisture contacting the feed tip and can also lead to accumulation of fragments of oxide and other debris between the feed tip and the rolls. Because of the curvature of the faces of the feed tip, the presence of such debris is not easily detected and can result in imperfections in the sheet formed by the continuous caster.
For similar reasons the inside faces of the continuous caster tip are formed with parallel surfaces connected by steep ramps. The width of the gap between the inside faces through which molten metal flows therefore has relatively rapid changes in cross-sectional area. This results in rapid changes in molten metal velocity and the changes in shape of the inside faces can provide locations for temporary accumulations of oxide and other debris which intermittently break loose and cause imperfections in sheet made by the continuous caster.
It is therefore desirable to provide a molten metal feed tip for a continuous caster having improved strength and ease of fabrication. The tip should be free from health hazards of asbestos and relatively insensitive to degradation by moisture. It is desirable for the feed tip to have smooth continuous contours to minimize defects in the sheets made by the continuous caster and avoid discontinuities in the properties of the feed tip. Preferably the molten metal feed tip should be reusable for several runs of the continuous caster.